1. Field of the Invention
This invention pertains to a method of preparing mixed zinc dialkyldithiophosphate compositions which are not prone to precipitation under normal storage conditions. In particular, this invention is directed to the addition of an ammonium salt of an acid during the preparation of the zinc compositions of this invention. The ammonium salt substantially retards the susceptibility of the mixed zinc dialkyldithiophosphate compositions to form a precipitate, thus improving its solution stability.
2. Description of the Prior Art
Zinc salts of dialkyldithiophosphoric acids are known to inhibit the oxidation of the lubricant oil while improving the anti-corrosion property of the lubricant oil composition. It is also known that the alkyl groups of these zinc dialkyldithiophosphates may be of high molecular weight or low molecular weight. Zinc dialkyldithiophosphates wherein the alkyl groups are of four carbon atoms or less are considerably less expensive than the zinc dialkyldithiophosphates containing alkyl groups derived from alcohols having five or more carbon atoms. In spite of this, these zinc dialkyldithiophosphates containing only alkyl groups derived from alcohols having four or fewer carbon atoms have not been widely marketed as lubricating oil additives. Reasons for this have been that these zinc di(loweralkyl)dithiophosphates are insufficiently oil soluble to allow preparation of a concentrate. Moreover, the normally liquid compositions of the low molecular weight zinc dialkyldithiophosphates are prone to precipitation (crystallization).
Normally, the zinc dialkyldithiophosphate additives of this invention should be oils and are transported either neat, that is only as the zinc dialkyldithiophosphate, or as a lubricating oil concentrate. In any event, precipitation of a portion of the additive during transport requires that the precipitate be solubilized prior to formulation.
In order to overcome this problem, U.S. Pat. Nos. 2,680,123; 3,151,075; 3,000,822; 3,385,791 and others teach the use of zinc dialkyldithiophosphates prepared from a mixture of a low molecular weight alcohol and a high molecular alcohol. This results in mixture of products containing a statistical distribution of pure zinc di(loweralkyl)dithiophosphate, zinc mixed-dialkyldithiophosphates and pure zinc di(higheralkyl)dithiophosphate as shown in the formula below: ##STR1## wherein R may be either a lower or higher alkyl group.
The use of the mixed alcohols improves the oil solubility of the resulting product while also lowering its overall costs.
However, as the mole percent of the lower alcohol to the total alcohol employed in preparing the mixed dialkyldithiophosphate exceeds approximately 50 percent, problems with the solution stability of the resulting zinc dialkyldithiophosphate composition are raised again. This problem results in a precipitate forming in the zinc dialkyldithiophosphate solutions. This problem of precipitation in mixed zinc dialkyldithiophosphates of high lower alkyl content is most apparent when the lower alkyl methyl, ethyl, n-propyl; and isopropyl groups are employed but also arises when isomers of butyl alcohol are employed. This is particularly evident when the amount of butyl alcohol used to form the mixed zinc dialkyldithiophosphate is raised to approximately 75-80 mole percent of the total alcohol content.
Without being limited to any particular theory, it is believed that, as the level of the lower alcohol employed to form the mixed zinc dialkyldithiophosphate is raised, the statistical distribution of products favors greater amounts of pure zinc di(loweralkyl)dithiophosphate (wherein all the R groups in FIG. 1 are the same lower alkyl group). These pure zinc di(loweralkyl)dithiophosphates are known to be prone to crystallization in solution and to be both insoluble in oil. Thus, although the use of a lower alcohol is economically favorable, the use of increasing the amounts of the lower alcohol results in solubility and crystallinity problems in the product.
Accordingly, the use of a mixture of lower and higher alcohols in preparing mixed zinc dialkyldithiophosphates is limited to mole ratios of lower to higher alcohols which favor a low statistical proportion of pure zinc di(loweralkyl)dithiophosphate. Nevertheless, the use of a maximum amount of the lower alcohol is economically advantageous and thus a solution which alleviates the problems of precipitation and solubility associated with the use of large amounts of the lower alcohol would be particularly beneficial.
This invention is directed to a method of preparing mixed zinc dialkyldithiophosphate compositions which are not prone to precipitation under normal storage conditions.
I have now found that precipitation (crystallization) on mixed zinc dialkyldithiophosphate compositions prepared from the reaction of phosphorus pentasulfide with a mixture of about 50-80 mole percent of a lower alcohol plus 50-20 mole percent of a higher alcohol followed by neutralization of the resulting dialkyldithiophosphoric acid with a basically reacting zinc compound may be alleviated by the addition to the reaction mixture of an effective amount of an ammonium salt to prevent precipitation of the resulting mixed zinc dialkyldithiophosphate. The use of an ammonium salt is also effective in preventing the precipitation of a mixed zinc dialkyldithiophosphate in an oil concentrate.
U.S. Pat. No. 4,085,053 discloses that metal dithiophosphates may be prepared by the neutralization of dithiophosphoric acid with a metal base in the presence of an acidic promoter followed by reacting a substantial portion of excess acidic promoter with a weak base. Among others this reference discloses acetic acid as an example of a promoter in the preparation of the metal dialkyldithiophosphates and ammonia (ammonium hydroxide) as an example of a weak base.
The addition of alkylamines or arylamines to a lubricating oil composition containing a zinc dithiophosphate is taught by U.S. Pat. Nos. 2,766,207; 3,284,354; and 3,018,247 among others.
U.S. Pat. No. 3,826,745 discloses amine salts of mixed acid phosphates as useful in inhibiting haze or precipitation formation in zinc dialkyldithiophosphates caused by small amounts of water.
U.S. Pat. No. 4,377,527 and Belgium Application No. 894,196 disclose the use of an ammonia-producing compound (such as ammonium salts of mineral acids and organic acids) as a catalyst in the neutralization reaction between a dihydrocarbyl dithiophosphoric acid and a basically reacting zinc compound. This reference further teaches that suitable ammonium producing species are those which will decompose to ammonia or ammonium and includes, for example, ammonium acetate.